Why do you need two aliases? I'd elimnate one and just set up port 5001 or 5002. If you're streaming on your LAN you shoudn't need to open a port, are you maybe running a 2nd subnet? If so that can cause havoc for your Slingbox. If your Slingbox is connected correctly when you start the software it will notice their is a Sling on your local network. I think you have just added too many steps for your setup and that's causing your issues.
On the lan 2 slings can use the same port on the WAN they cannot. Simply change the second slings port to something other than 5001 create a port forward to that lan ip and port
Here is why its not working
On a remote connection your sling is your ip and port. Say 10.10.10.250 is your ip 10.10.10.250:5001 is your unique internet address for that sling. Your router forwards any traffic on port 5001 to the sling lan ip. When you have 2 slings you need a unique internet address. so anything but 10.10.10.250:5001 will work